1. Field of the Invention
The invention relates generally to optical waveguides, optical waveguide components for signal processing and optical waveguide sensors for various physical fields, and more specifically to multimode optical waveguides having a specific refractive index profile of their core providing a possibility to guide a higher order mode with sharp central peak of its field and to optical components and sensors based on this waveguide designed for optical fiber communication systems, sensor systems and other optical devices for industrial and medical applications.
2. Information Disclosure Statement
Integrated optical waveguides and optical fibers are key components of optical fiber communication systems, optical sensors and other optical devices for various industrial and medical applications. All these components and sensors essentially consist of an optical waveguide having a core region and outer cladding with lower refractive index. Many types of various optical waveguide components and sensors have been developed for different industrial and medical applications.
The most sensitive sensors and the most commonly used waveguide components for optical fiber communication systems usually employ single-mode waveguides or fibers since these waveguides provide good compatibility with typical single-mode fiber network and reduced modal noise. Employing the single-mode waveguide in optical sensors provides a possibility to design interferometric sensors which are much more sensitive than the sensors of amplitude type. The multimode waveguides having larger cross section than single-mode waveguides, however, are basically more convenient to deal with in practice. In some specific applications such as chemical sensors based of Fourier spectroscopy in middle infrared range of spectrum, moreover, one must use multimode fibers since single-mode fibers operating in middle infrared are not available. The modal noise of speckle pattern resulting form the interference of many modal fields in a multimode waveguide is the main problem to be overcome in order to employ these waveguides especially in interferometric sensors and components. Therefore, multimode waveguides are usually used only in sensors of amplitude type where the sum of intensities of all waveguide modes is detected. Sensitivity of such sensors, however, is much lower than the sensitivity of single-mode interferometric sensors. Therefore it is very important to design highly sensitive sensors and components based on multimode rather than on single-mode waveguides. This problem can be solved by providing a single-mode regime of operation in the multimode waveguide since each selected mode is more sensitive to external influence to be detected than whole set of the modes carried by the waveguide.
It have been proposed to use computer generated holograms for selective excitation and detection of some specific modes inside a multimode fiber in order to realize sensors with enhanced and tunable sensitivity by switching from one operating mode to another. This method being very attractive from physical point of view is practically not very convenient since it is difficult to integrate the computer generated holograms in a standard single-mode optical fiber network. It is desirable to have a possibility of creating a single mode regime of operation in a multimode waveguide without employing the computer generated holograms, which usually also require complicated input-output optics.